1. Field of the Invention
The present invention relates to wellbores, and more specifically, to methods and apparatus for selectively isolating areas such as production zones within the wellbore.
2. Description of the Related Art
After a well has been drilled and the casing is cemented into it, one or more sections of the casing are perforated. These perforations allow for the in-flow of fluids from the formation at one or more subterranean production zones. A production tubing is then inserted and installed into the well for the purpose of conducting fluids from the production zone to the surface.
Sometimes, during the life of the well, the perforations in a particular section of the casing may begin to admit an unacceptable level of contaminants. For example, in the case of an oil well, unacceptable levels of water or sand may enter the wellbore from one or more production zones. Those production zones producing the excessive amount of water or sand must then be plugged or otherwise sealed to prevent the continued entry of water or sand into the wellbore. In some cases, the plugged well still produces fluid from other production zones, while in other cases all production from the well is shut down.
One technique commonly used to seal the unwanted zones involves the creation of a cement plug or lining on the inside surface of the casing. This technique normally requires removal of the production tubing from the well followed by insertion of a temporary packer to isolate the unwanted zone. Cement is then poured down the wellbore on top of the packer and, after curing, forms the desired plug to seal the perforations in the unwanted zone. To restore production from the lower zones of the well, the center of the cement plug must be bored out. Sealing unwanted zones using cement, however, has often proved unsatisfactory because the cement tends to crack and permit water to leak into the production field. Additionally, the removal and reinstallation of the production tubing requires significant time, effort, and cost.
Another prior art technique for sealing of portions of wellbore casings is disclosed in U.S. Pat. No. 5,833,001 issued to Song et al. (“Song”). Song discloses an inflatable downhole device that installs a composite sleeve on the inner surface of a damaged casing at a particular depth. Uncured composite material comprising an epoxy layer having a mixture of resin and a curing agent surrounded by a sealing film are disposed on the outside of the inflatable device. The device is then lowered down through the production tubing to the desired depth. Once at this depth, the device is inflated to press the composite materials against the inner diameter of the casing. The composite materials are then heated under pressure to form a sealing liner which closes off the perforations in the damaged casing.
Expandable steel casings have also been used to restore wellbore integrity when existing casing strings have become damaged or severely corroded. These types of steel casings may provide some advantages where there are high differential pressures across the casing and a high strength liner is necessary. The casings however, are intolerant of changes in wellbore diameter and may become stuck during deployment. The use of these steel casings can involve significant effort and expense because the production tubing has to be pulled from the well before the expandable steel casing and installation equipment can be run down the hole.
Deficiencies also exist in the known methods of plugging a well where it has become damaged beyond repair or has reached the end of its useful life. Before a well is abandoned, state and federal regulations frequently require it to be plugged for safety and environmental purposes. Typically, the well is plugged by simply pumping cement into the wellbore and allowing it to cure. Cement plugs like cement liners, however, have been known to crack and allow fluid to leak through the plug.
Bentonite has also been used in another prior art method for plugging an abandoned well. In such cases, water is poured down the wellbore. Next, hygroscopic bentonite pellets are dropped down the wellbore along with alternating levels of gravel. The bentonite pellets are hydrated by the water, which causes the bentonite to expand and thus seal the well. The hydration of the pellets, however, is oftentimes uncontrollable. For example, when the pellets are dropped down the wellbore they may stick to the sides of the casing or other equipment, and prematurely hydrate, thus clogging the wellbore and preventing effective sealing of the well.
Wireline inflatable plugs have been used to isolate intervals in the wellbore without having to pull the production tubing. Inflatable wireline plugs are not capable of withstanding high differential pressures without making additional runs to dump cement on top of the plug. The inflatable packer used also requires substantial metal reinforcement to withstand even low differential pressures. This makes it difficult to remove or mill up the plug if it is necessary to reenter the interval below the packer.
Therefore, a need exists for a method and apparatus for repairing or plugging a wellbore without having to remove the production tubing and without having the disadvantages associated with conventional methods and apparatuses.